The invention concerns a process for producing pistons for a reciprocating internal combustion engine, especially an Otto engine with direct injection and especially a motor vehicle engine, in which e piston is produced by a casting process.
In the manufacture of reciprocating internal combustion engines, variations in pistons produced by casting lead to corresponding deviations in the desired piston displacement and the compression ratio xcex5. For example, for xcex5=11, tolerances of 0.5 are normal. In the case of pistons with a smooth surface of the face of the piston that faces the combustion chamber, tolerances from the casting process can be compensated by mechanical finishing. However, this is no longer possible in the case of pistons with a fissured surface, which encompass, for example, depressions and elevations for suitably influencing an injection process in Otto engines with direct injection. In pistons of this type, a significant portion of the surface of the piston that faces the combustion chamber remains as a cast surface. Therefore, the volume in the cylinder that is determined by the surface of the piston facing the combustion chamber is subject to large variations corresponding to the large tolerances in the casting process. However, this has an unfavorable effect on engine function. For example, exhaust gas temperature, pollutant emission, and fuel consumption may rise.
Therefore, the goal of the present invention is to develop a process of the type referred to above, in which the cited disadvantages are overcome, and the tolerances for piston displacement and compression ratio are reduced to the smallest possible values.
To this end, the invention provides that, at least for the first piston of a casting process batch, the volume Vactual formed by the surface of the piston that faces the combustion chamber is determined, that this volume Vactual is compared with the desired nominal volume Vnominal, and that, based on this comparison, the compression height KH is fixed in such a way that its deviation from a set point ensures that, subsequently, Vactual=Vnominal within tolerance limits.
This has the advantage of achieving very small variation with respect to the compression ratio, since, for each batch, the piston pin bore is formed accordingly, i.e., at a different place in the piston, so that any deviations that may be present after the casting process due to variations in the volume of the piston defined by the combustion-chamber face are compensated with the compression height.
After a batch of pistons has been cast, it is advantageous to form the piston pin bore in all of the pistons at a predetermined distance KH from the surface of the pistons. This is done by machining the pistons, e.g., by milling.
In a preferred embodiment, the volume is determined by immersing the piston with its combustion-chamber face into a liquid cushion and measuring the displacement, which is then used to calculate the volume of the combustion-chamber face.
The pistons produced in the casting process have a fissured surface or depressions and elevations of the face of the piston that faces the combustion chamber. Therefore, a significant portion of the surface of the piston that faces the combustion chamber cannot be suitably mechanically finished, so this surface remains a rough, unfinished cast surface.